In electrophotography, an electric latent image is formed on an image carrier and is developed by toner to form a toner image, which is then transferred onto a fixing sheet, such as a sheet of paper, and is then fixed by means of heating and pressing, etc. For the toner to be used for forming images, not only conventional monochromatic black toner, but also multiple-colored toners for forming full-color images, are used to form the images. The toner image transferred onto the fixing sheet is fixed on the fixing sheet under heating and pressing while passing between a toner-heating member, such as a heat roller or a heat belt, and a press roller functioning as an oppositely disposed pressure-support member, and is discharged together with the fixing sheet out of the image forming apparatus.
Recently, in response to demand for a higher speed operation of electrophotographic image forming apparatus, there arise a demand for a further speed increase in a fixing device included in the image forming apparatus and a demand for reducing the time taken to reach a printable state (ready state) from power switch-ON for starting or switch-ON in a low-power consumption (power-saving) mode (that is, preheating mode or sleep mode). On the other hand, in the fixing device that occupies a substantial portion of the energy consumed by the electrophotographic image forming apparatus, there is a demand for suppressing the total energy consumption and achieving the speed increase in combination.
Further, although it has been conventional to supply heating energy to the toner image form only a toner heating member, it has become a recent practice to supply heating energy to a press roller, in addition to the toner heating member, in accordance with demand for higher speeds and an increasing demand for full-color image formation which requires increased thickness of the toner layer on the fixing sheet. In these circumstances, particularly, in order to achieve both suppression of energy consumption and reduction in the time taken to reach a ready state for the fixing device, there have been proposed a fixing device including a control unit that variably controls the power supply to a toner heating member, such as a heat roller, and a press roller (JP-A-2008-268957), and a fixing device including a control unit that variably controls the pre-run speed of a toner heating member and a press roller (JP-A-2009-301028). Although the fixing devices are advantageous in achieving the objects described above, the fixing devices have a drawback that the control units are complicated.